WASHINGTON—Angus King of Maine ran as an independent for the US Senate, but on Wednesday said he would caucus with Democrats, giving the party a 55-45 majority.

King’s decision was widely expected, but the former two-term governor insisted that he would not be bound by partisanship and would remain an independent voice for his state.

“By associating myself with one side, I am not in automatic opposition to the other,” King said in a press conference outside the Senate floor.

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Senate Majority Leader Harry Reid of Nevada later joined King at the podium to welcome him into the caucus, saying that he hoped King “will be a bridge to working with the Republicans.”

Afterward, the two men strode into a Senate hall for an organizing caucus. King is one of a dozen new members of the upper chamber of Congress, most of them Democrats, including Elizabeth Warren of Massachusetts.

On the campaign trail, King had declined to say which caucus he would join. Despite being coy, it was widely expected he would join the Democratic caucus, which currently has two independents among its ranks.

Senator Bernie Sanders, a liberal from Vermont, was easily reelected last Tuesday and will again join the party. Retiring Senator Joseph Lieberman of Connecticut served as an independent in his final term, after a long political career as a Democrat.

King said he relied on the advice of Lieberman and Sanders before announcing his decision.

“In answering this question repeatedly during the campaign,” King said, “I established two basic criteria—that I wanted to maintain my independence as long and as thoroughly as possible while at the same time being effective in my representation of Maine.”

King said he thought about going at it alone, but ruled it out because he would be largely excluded from the crucial work in committees. “Occasionally, my vote would prove crucial and be eagerly sought by both sides, but in the long run, I would be relegated to the sidelines as the day-to-day work of the Senate was done by others.”

Last week’s election made it easy to choose sides, he said. “In the situation where one party has a clear majority and effectiveness is an important criteria, affiliating with the majority makes the most sense. The majority has more committee slots to fill, has more control over what bills get considered, and more control over the Senate schedule.”

King said he sought a seat on the Finance Committee, but acknowledged it was a long shot.

King said he received no overtures from Senate Minority Leader Mitch McConnell to join the Republican caucus, but did meet with Senator Roy Blunt of Missouri.